ArcAdiAThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.http://dspace-roma3.caspur.it:802015-08-02T22:26:58Z2015-08-02T22:26:58ZAb initio investigation of the structural and electronic properties of tio2 nanostructuresIacomino, Amilcarehttp://hdl.handle.net/2307/6082011-09-26T23:36:51Z2010-01-19T23:00:00Z<Title>Ab initio investigation of the structural and electronic properties of tio2 nanostructures</Title>
<Authors>Iacomino, Amilcare</Authors>
<Issue Date>2010-01-20</Issue Date>
<Abstract>In the ﬁrst part of this thesis, we have investigated the structural and electronic properties of anatase TiO2 nanocrystals (NCs) through ab initio calculations based on the Density
Functional Theory (DFT). The dependence of the structural properties (e.g., NC volume
variations) on the surface chemistry is discussed by considering two diﬀerent surface coverages (dissociated water and hydrogens). Both prevent a pronounced reconstruction of the
surface, thus ensuring a better crystalline organization of the atoms with respect to the bare
NC. In particular, the results for the hydrated NC do show the largest overlap with the
experimental ndings. The band-gap blue shift with respect to the bulk shows up for both
the bare and the hydrated NC, whereas hydrogen coverage or oxygen desorption from the
bare NCs induce occupied electronic states below the conduction levels thus hindering the
gap opening due to quantum connement. These states are spatially localized in a restricted
region and can be progressively annihilated by oxygen adsorption on undercoordinated surface titanium atoms. Formation energy calculations reveal that surface hydration leads to
the most stable NC, in agreement with the experimental ndings that the ﬁrst layer of the
water coverage is important in stabilizing the NCs surfaces. Oxygen desorption from the
bare NC is unfavourable, thus highlighting the stabilizing role of surface oxygen stoichiometry for TiO2 . Available experimental data on the electronic and structural properties of
TiO2 NCs are summarized and compared with our results.
In the second part of this thesis, we have performed DFT calculations on anatase TiO2
nanowires (NWs) to investigate the dependence of their structural and electronic properties
on the size, the surface coverage, and the morphology. We have found that the overall
crystallinity of the NWs increases on increasing the diameter size or upon surface coverage
with simple water-derived adsorbates. The NWs grown along the [010] direction are found
to be more ordered with respect to the NWs in the [001] direction, thus highlighting the
dependence of the crystallinity on the choice of the morphology. The bare and hydrated
NWs do show the band-gap blue shift due to the size connement, but deviations from an
ideal trend with the size are found and ascribed to the morphology and the crystallinity.
Through the analysis of the valence band maximum and conduction band minimum energies
we found that the TiO2 NWs photocatalytic ability prots from the conned size, for example
2
for the water splitting process. Moreover, the availability of internal channels for an efcient
charge transport can be tuned by the surface coverage. The terminal hydroxyl groups of the
hydrated NWs cannot be considered as deep hole traps since their related electronic states
have binding energies in the same range of the NW oxygen states. The hydrogenated NWs
grown along the [001] direction show occupied states at the bottom of the conduction bands,
thus we expect that TiO2 NWs can be used as efcient hydrogen sensors. Finally, the surface
hydration leads to the most stable NWs with formation energies that are even close to the
bulk limit.</Abstract>2010-01-19T23:00:00Z